1. Technical Field
The present invention relates to wireless broadcast systems and, in particular, to multiple-input, multiple-output wireless broadcast systems having a constraint on the number of streams that can be transmitted to a user.
2. Description of the Related Art
As the number of mobile wireless users increases, and as the quality of services demands rise, higher data rates are needed to meet demand. In current wireless networks download transfer rate is of primary interest, such that one or a group of base stations should be able to simultaneously transmit data to a group of users, satisfying individual instantaneous or average target bit rates.
Current wireless systems are often based on having a single user transmission per resource block. For example, each user may receive an independent portion of time-frequency resources. However, the use of multiple-input multiple-output (MIMO) antennas in single-user and point-to-point communication systems results in a multiplexing gain in fading channels, scaling up the system throughput linearly with the lesser of the number of antennas at the transmitter and the antennas at the receiver.
Existing technologies are based on a single stream transmission. However, the use of multiple streams in a MIMO broadcast channel can increase individuals' achievable rates. Even proposed standards consider only very limited numbers of streams transmitted to each user. The capacity of a multi-user MIMO system is enhanced by the use of multiple streams and successive interference precancellation techniques called dirty paper coding (DPC). Realization of DPC in practice has been through non-linear precoding approaches and is generally complicated and costly. As such, few practical systems employ DPC. Whether or not DPC is used, an increase in the number of streams increases base station and receiver complexity. As such, little effort has been made to optimize the use of multiple streams and DPC.